@spec get_revenue_tracking_currency(Plausible.Site.t(), Plausible.Stats.Query.t(), [
  atom()
]) ::
  {atom() | nil, [atom()]}

Returns the common currency for the goal filters in a query. If there are no goal filters, or multiple currencies, nil is returned and revenue metrics are dropped.

Aggregating revenue data works only for same currency goals. If the query is filtered by goals with different currencies, for example, one USD and other EUR, revenue metrics are dropped.

__internal_visits is fetched when querying bounce rate and visit duration, as it is needed to calculate these from imported data. This function removes that metric from all entries in the results list.